Fire crews were called to the scene of an explosion at a site belonging to BAE Systems, Britain's biggest defense firm.
South Wales Fire and Rescue Service said the incident happened at the BAE base in Glascoed, Monmouthshire, at around 10.49am on Wednesday, 17 April.
In a tweet on X, the fire service said an "incident control post has been set up to deal with the incident".
A BAE Systems spokesperson said, "We can confirm that there was an incident at our Glascoed facility in Wales this morning.
"Tried and tested safety protocols were immediately enacted and emergency services attended the site.
"All personnel are accounted for and there were no injuries. Our emergency response team continue to monitor the facility and a full investigation has been launched."
The weapons factory site in Glascoed was built in 1938 and established to help prepare the Royal Navy ahead of the Second World War. It now employs around 500 people.
MNA